It is well known to include in various kinds of imaging elements, a transparent layer containing ferromagnetic particles dispersed in a polymeric binder. The inclusion and use of such transparent magnetic recording layers in light-sensitive silver halide photographic elements has been described in U.S. Pat. Nos. 3,782,947; 4,279,945; 4,302,523; 4,990,276; 5,215,874; 5,217,804; 5,229,259; 5,252,441; 5,254,449; 5,395,743; 5,413,900; 5,427,900; 5,498,512; and others. Such elements are advantageous because images can be recorded by customary photographic processes while additional information can be recorded simultaneously into or read from the magnetic recording layer by techniques similar to those employed in traditional magnetic recording art.
Any transparent magnetic recording layer must be capable of accurate recording and playback of digitally encoded information repeatedly on demand by various devices such as cameras, photofinishing equipment or printing systems. The magnetic recording layer also must exhibit excellent runnability, durability (i.e., abrasion and scratch resistance), and magnetic head-cleaning properties without adversely affecting the imaging quality of the photographic element. However, this goal is difficult to achieve because of the nature and concentration of the magnetic particles required to provide sufficient signal to write and read magnetically stored data and the effect of any noticeable color, haze or grain associated with the magnetic layer on the optical density and granularity of the photographic imaging layers. These requirements are particularly difficult to meet when magnetically recorded information is stored and read from the photographic image area. Further, because of the curl of the photographic element, primarily due to the photographic emulsion layers and the core set of the support, the magnetic layer must be held more tightly against the magnetic heads than in conventional magnetic recording in order to maintain planarity at the head-media interface during recording and playback operations. Thus, all of these various requirements must be considered both independently and cumulatively in order to arrive at a commercially viable photographic element containing a transparent magnetic recording layer that will not have a detrimental effect on the photographic imaging performance and still withstand repeated and numerous read-write operations by a magnetic head.
Electrostatic charge can be generated during the use of an imaging element incorporating a transparent magnetic recording layer, such as photographic film. For example, in an automatic camera, because of the repeated motion of the photographic film in and out of the film cassette, electrostatic charge can be generated by the movement of the film across the magnetic heads and by the repeated winding and unwinding operations, especially in a low relative humidity environment. The accumulation of charge can result in the attraction and adhesion of dust to the film surface. The presence of dust can result in physical defects, the degradation of the image quality of the photographic element as well as the degradation of magnetic recording performance (e.g., decreased S/N ratio, "drop-outs", etc.). Degradation of magnetic recording performance can arise from various sources including signal loss caused by increased head-media spacing or clogging of the magnetic head gap, electrical noise from static discharge by the magnetic head during playback, uneven film transport across the magnetic heads, and excessive wear of the magnetic heads. In order to minimize problems caused by electrostatic charge during the manufacture and use of imaging elements incorporating a magnetic recording layer, an electrically-conductive antistatic layer can be incorporated into the imaging element in various ways to dissipate any accumulated static charge, for example, as a subbing layer, an intermediate layer, and especially as an outermost layer either overlying the imaging layer or as a backing layer on the opposite side of the support from the imaging layer(s). Typically, in photographic elements containing a transparent magnetic recording layer, the antistatic layer is located underlying the magnetic recording layer as a backing layer.
A wide variety of conductive antistatic agents can be incorporated in antistatic layers to produce a broad range of surface electrical conductivities. Many of the conventional antistatic layers used in photographic elements employ materials which exhibit predominantly ionic conductivity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, clay sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like have been taught in prior art. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge-carrying ions are greatly reduced and the bulk conductivity is substantially decreased. Further, at high relative humidities, an unprotected antistatic backing layer can absorb water, swell, and soften. Especially in the case of roll films, this can result in the adhesion (viz., ferrotyping) and even physical transfer of portions of a backing layer to a surface layer on the emulsion side of the film (viz., blocking). Because of the aqueous solubility and ion-exchange properties of such ionic conducting materials, unprotected antistatic layers containing these materials typically do not exhibit acceptable antistatic properties after photographic processing.
U.S. application Ser. No. 08/937,685 (filed Sep. 29, 1997) now U.S. Pat. 5,891,611, assigned to the same assignee as the present application and incorporated herein by reference discloses a polyolefin resin-coated photographic paper having a print-retaining antistatic backing layer containing an electrically-conducting swellable or smectite clay and a polymeric binder wherein the polymeric binder is capable of sufficiently intercalating inside or exfoliating the smectite clay. One particularly preferred type of swellable or smectite clay is a synthetic layered hydrous magnesium silicate resembling the natural clay mineral hectorite in both structure and chemical composition. Polymers capable of intercalating in such a clay are those which are sorbed between the silicate platelets of the clay particles so as to cause a separation or an increase in separation between adjacent silicate platelets. The extent of intercalation or exfoliation of the clay particles by the polymer can be conveniently monitored by measuring the basal (001) spacing of the clay platelets using an x-ray diffraction technique, as described by Gianellis et al. in U.S. Pat. No. 5,554,670. Typically, when intercalation occurs, an increase in the basal spacing of the clay is observed. When exfoliation occurs the diffraction peaks broaden so as to disappear altogether, since long-range crystallographic order is lost. Suitable polymers that provide conductive backing layers with desirable performance characteristics for photographic paper are disclosed in the '685 application and include water soluble polymers such as polyvinyl alcohols, polyethylene oxides, polyacrylamides, polystyrene sulfonates, water-insoluble polymers such as polymers of styrene, polymers of styrene derivatives, interpolymers of styrene, interpolymers of styrene derivatives, alkyl acrylates, alkyl methacrylates, derivatives of alkyl acrylates, derivatives of alkyl methacrylates, olefins, acrylonitriles, polyurethanes and polyester ionomers. Other suitable polymers that can sufficiently intercalate in or exfoliate smectite clays are disclosed in U.S. Pat. No. 5,552,469.
Antistatic layers containing electronic conductors such as conjugated conductive polymers, conductive carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films can be used more effectively than ionic conductors to dissipate static charge since their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors, electrically-conductive metal-containing particles, such as semiconductive metal oxides, are particularly effective when dispersed with suitable polymeric film-forming binders in combination with polymeric non-film-forming particles as described in U.S. Pat. Nos. 5,340,676; 5,466,567; 5,700,623. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example: U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; 5,484,694; and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include Sb-doped tin oxide, F-doped tin oxide, Al-doped zinc oxide, and Nb-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other conductive metal-containing granular particles including metal borides, carbides, nitrides, and silicides have been disclosed in Japanese Kokai No. JP 04-055,492.
The use of such electrically-conductive layers containing suitable semiconductive metal oxide particles dispersed in a film-forming binder in combination with a transparent magnetic recording layer in silver halide imaging elements has been described in the following examples of the prior art. Photographic elements containing a transparent magnetic recording layer and a transparent electrically-conductive layer both located on the backside of the film base have been described in U.S. Pat. Nos. 5,147,768; 5,229,259; 5,294,525; 5,336,589; 5,382,494; 5,413,900; 5,457,013; 5,459,021; 5,707,791, and others. The conductive layers described in the cited patents contained fine granular particles of a semiconductive crystalline metal oxide such as zinc oxide, titania, tin oxide, alumina, indium oxide, silica, complex or compound oxides thereof, and zinc antimonate or indium antimonate dispersed in a polymeric film-forming binder. Of these conductive metal oxides, antimony-doped tin oxide and zinc antimonate are preferred. A granular, antimony-doped tin oxide particle commercially available from Ishihara Sangyo Kaisha under the tradename "SN-100P" was disclosed as particularly preferred in Japanese Kokai Nos. 04-062543, 06-161033, and 07-168293. Surface electrical resistivity (SER) values were reported in U.S. Pat. No. 5,382,494 for conductive layers measured prior to overcoating with a transparent magnetic recording layer as ranging from 10.sup.5 to 10.sup.7 ohms/square and from 10.sup.6 to 10.sup.8 ohms/square after overcoating. Surface resistivity values of about 10.sup.8 to 10.sup.11 ohms/square for conductive layers overcoated with a transparent magnetic recording layer were reported in U.S. Pat. Nos. 5,457,013 and 5,459,021.
The use of colloidal, electrically-conductive metal antimonate particles (e.g., zinc antimonate) in antistatic layers for imaging elements, especially for silver halide-based photographic elements, is broadly claimed in U.S. Pat. No. 5,368,995. Further, the use of colloidal, conductive metal antimonate particles in antistatic layers in combination with a transparent magnetic recording layer is taught in U.S. Pat. No. 5,457,013. However, dry weight coverages of metal antimonates in conductive subbing and backing layers sufficient to provide preferred levels of electrical conductivity for antistatic protection of imaging elements produce an undesirable increase in optical density because of absorption and haze due to scattering by agglomerates of particles. The requirements for low optical density, low haze, lack of photoactivity, and low manufacturing cost dictate that such a conductive layer must be coated using as low a dry weight coverage of metal antimonate as possible. Further, for the conductive layers disclosed in the '013 patent containing less than about 85% zinc antimonate by weight, the internal resistivity of the conductive layer increased appreciably after overcoating with a transparent magnetic recording layer.
An imaging element consisting of a support, at least one image-forming layer, a transparent magnetic recording layer, and at least one transparent, electrically-conductive layer, wherein the electrically-conductive layer contains both colloidal electrically-conductive metal antimonate particles and colloidal, non-conductive, metal-containing filler particles of comparable or smaller size, and one or more film-forming polymeric binders is disclosed in copending U.S. application Ser. No. 08/970,130 (filed Nov. 13, 1997) assigned to the same assignee as the present application and incorporated herein by reference. A wide variety of non-conductive metal-containing filler particles can be substituted for the conductive metal antimonate particles. Suitable non-conductive filler particles include metal oxides, clays, proto-clays, clay-like minerals, zeolites, micas, talcs, and the like. Particularly suitable non-conductive filler particles include colloidal (e.g., .about.0.002-0.050 .mu.m) particles of non-conductive tin oxide, zinc oxide, antimony pentoxide, zinc antimonate, silica, alumina-modified silica, various natural clays, synthetic clays, and the like. Such filler particles can be substituted for up to about 75% of the conductive metal antimonate particles without an appreciable decrease (i.e., .ltoreq.1 log ohm/square) in the surface electrical conductivity of the conductive layer.
A three-component antistatic layer which includes a smectite clay, one or more additive(s) or polymeric binder(s) which can sufficiently intercalate in or exfoliate the smectite clay, and one or more film forming polymeric binders which do not sufficiently intercalate in and/or exfoliate the conducting smectite clay is disclosed in U.S. application Ser. No. 08/940,860 (filed Sep. 29, 1997) assigned to the same assignee as the present application and incorporated herein by reference. Polymeric binders which do not intercalate or exfoliate the smectite clay can still be incorporated in a functional antistatic layer, by the inclusion of one of the polymeric binders disclosed therein which can sufficiently intercalate in and/or exfoliate the smectite clay as a co-binder. This allows a wider selection of polymeric binders with different physical and chemical characteristics to be used in antistatic layers while maintaining the benefits provided by the polymer-intercalated clay filler particles. The overall performance of the three-component conductive layers described in the '860 application include improved surface resistivity, backmark retention, splice strength, and trackoff for paper supports. However, one noteworthy deficiency of conductive layers containing clay, intercalated clay or exfoliated clay as the conductive filler is that unless the conductive layer is overcoated with a non-permeable protective layer such as poly(methylmethacrylate), for example, the conductivity of the layer is substantially diminished by photographic processing. The use of such a polymer intercalated or exfoliated smectite clay in combination with electronically-conductive metal-containing colloidal particles, such as antimony-doped tin oxide or zinc antimonate, in a transparent, processing-surviving, antistatic layer underlying a transparent magnetic recording layer in imaging elements was neither disclosed nor anticipated by the '860 application.
Because the requirements for an electrically-conductive layer to be useful in an imaging element are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a balance of the necessary chemical, physical, optical, and electrical properties. As indicated hereinabove, the prior art for providing electrically-conductive layers useful for imaging elements is extensive and a wide variety of suitable electrically-conductive materials have been disclosed. However, there is still a critical need for improved conductive layers which can be used in a wide variety of imaging elements, which can be manufactured at a reasonable cost, which are resistant to the effects of humidity change, which are durable and abrasion-resistant, which do not exhibit adverse sensitometric or photographic effects, which exhibit acceptable adhesion to overlying and underlying layers, which exhibit suitable cohesion, and which are substantially insoluble in solutions with which the imaging element comes in contact, such as processing solutions used for silver halide photographic elements. One objective of this invention is to provide a conductive layer containing lower dry weight coverages of conductive metal-containing particles than those of prior art. Another objective of this invention is to provide a conductive layer which exhibits improved adhesion and minimal increase in resistivity when overcoated with a transparent magnetic recording layer relative to conductive layers of prior art. Furthermore, to provide both effective magnetic recording properties and effective electrical conductivity characteristics in an imaging element, without impairing the required imaging characteristics, poses an even greater technical challenge. Thus, it is toward the objective of providing a combination of a transparent magnetic recording layer and an underlying electrically-conductive layer that more effectively meet the diverse needs of imaging elements, especially those of silver halide photographic films as well as those of a wide range of other types of imaging elements, that the present invention is directed.